1. Field of the Invention
The present invention relates to an optical disc apparatus for reading out and reproducing information, such as audio and video, recorded on an optical disc such as DVD (Digital Versatile Disc). It more particularly relates to an optical disc apparatus which does not have, i.e. is absent of, a function of reproducing or outputting DTS (Digital Theater System) audio.
2. Description of the Related Art
Conventionally, optical discs such as DVD-ROM (DVD-Read Only Memory) with so-called multiplex audio streams are known, in which various compressed audio data, such as LPCM (Linear Pulse Code Modulation), Dolby Digital, MPEG (Moving Picture Coding Experts Group) audio and DTS, are recorded. In such kinds of optical discs, there is an increasing trend that a DTS audio is recorded in the first stream. On the other hand, some optical disc apparatus for reproducing information such as audio recorded on an optical disc do not have, in audio decoders thereof, any decoder for reproducing DTS audio (such decoder being hereinafter referred to as DTS decoder). Some other optical disc apparatus are so designed as to output no DTS audio because no patent license fees are paid for DTS decoders.
Such optical disc apparatus, which do not have, i.e. are absent of, a DTS audio reproduction function or DTS audio output function, are usually so designed that a user cannot select DTS audio reproduction. Furthermore, such optical disc apparatus are usually so set at the initial setting for factory shipment that other audio than the DTS audio is automatically selected at the first reproduction in use after a power supply is switched on. Accordingly, no compressed data of DTS audio is sent to an audio decoder, thereby preventing such state that no audio is output from an audio output terminal, i.e. preventing the optical disc apparatus from becoming unable to output audio from an audio output terminal.
Some of such kinds of optical disc apparatus, in which a priority language is set at the initial setting, are so designed that if compressed audio data of the priority language is recorded on the optical disc, audio is reproduced and output in accordance with the compressed audio data.
Japanese Laid-open Patent Publication Hei 10-125003 discloses an optical disc apparatus that uses a stream ID (identifier) and a substream ID for dividing various compressed data in order to handle auxiliary video data and various audio data in addition to MPEG video data.
Furthermore, Japanese Laid-open Patent Publication 2000-222826 discloses an optical disc apparatus that sets a signal amplifier circuit at mute state even in the case where the expanded signal from the compressed audio signal shows silence, thereby preventing the signal amplifier circuit from outputting noise, for the purpose of obtaining better S/N ratio when D/A converting compressed audio signals.
Further, in the field of audio reproduction apparatus having no multi-channel reproduction function and having only a stereo two-channel reproduction function, there is a case where audio recorded on an optical disc for reproduction comprises only a multi-channel signal without any coefficients for downmixing the multi-channel signal to a stereo two-channel signal. Japanese Laid-open Patent Publication 2001-344906 discloses an optical disc apparatus that, in the above case, downmixes the multi-channel signal to a stereo two-channel signal in accordance with downmix coefficients recorded on a ROM in the audio reproduction apparatus, in order to prevent the occurrence of a state such that no audio is output, i.e. prevent the optical disc apparatus from becoming unable to output audio.
Furthermore, in the field of audio transmitters and audio receivers, it may occur that when transmitted audio streams change in the kind, the starting or ending portion of audio reproduced at the receiver side cuts out. For the purpose of preventing such occurrence, when transmitted audio streams change in the kind, an audio transmitter disclosed in Japanese Laid-open Patent Publication 2001-250318 outputs, for a given time, a packet containing a silence identifier and an identifier information prior to the change together with a packet containing a silence identifier and an identifier information after the change.
According to the conventional optical disc reproduction apparatus as described above, which do not have, i.e. are absent of, a DTS audio reproduction function or DTS audio output function, the initial setting is made such that other audio than the DTS audio is automatically selected at the time of reproduction. However, when a user stops the reproduction process, the initial setting is canceled. Accordingly, in the case where the user once stops the reproduction process, and then restarts the reproduction without selecting the kind of audio, there is a possibility that a compressed data of DTS audio recorded in the first stream on the optical disc is sent to an audio decoder, whereby such state may occur that no audio is output from the audio output terminal. Such problem cannot be solved in the case of the above-described method of reproducing, with priority, compressed audio data of a priority language. Furthermore, such problem cannot be solved by any technologies according to the above-described four references.